Fix #337 Use the correct bash expansion in install-plugins
diff --git a/install-plugins.sh b/install-plugins.sh
index 722fb3f..8b8daee 100755
--- a/install-plugins.sh
+++ b/install-plugins.sh
@@ -62,7 +62,7 @@
         return 0
     fi
 
-    JENKINS_UC_DOWNLOAD=${JENKINS_UC_DOWNLOAD=-"$JENKINS_UC/download"}
+    JENKINS_UC_DOWNLOAD=${JENKINS_UC_DOWNLOAD:-"$JENKINS_UC/download"}
 
     url="$JENKINS_UC_DOWNLOAD/plugins/$plugin/$version/${plugin}.hpi"